(1) Field of the Invention
The present invention relates generally to the dyeing of textiles and, more particularly, to a liquid alkali for soaping off fiber reactive dyes from cotton and cotton blended fabrics.
(2) Description of the Prior Art
Fiber reactive dyes were first introduced in the mid 1950's. Since that time they nave become a dominant factor in dyeing cotton, regenerated cellulose and blends. These dyes can also be used to dye acrylics, nylon, silk, wool and blends of these fibers. Fiber reactive dyes are easy to apply and produce brilliant shades, fastness, penetration and leveling.
Fiber reactive dyes are anionic in nature and react chemically with the fiber. The dyes include a chromophore to give color to the dye and a reactive group to form a chemical bond with the fiber. There may also be a substitutent or solubilizing group which provides additional dyeing characteristics such as solubility, substantivity, migration, washing off, etc. Fiber reactive dyes react in the presence of alkali to form a strong covalent chemical bond between a carbon atom of the dye molecule and an oxygen atom of the hydroxyl group in the cellulose. This step is called "fixing".
Following the fixing step, the fabric is scoured to remove excess reactive dyes. This process is called "soaping off". Normally, soaping off includes a first rinse step, a soaping step, a second rinse step, and a drying step. Typically, soaps, such as phosphate-based detergents, are added during the soaping step to assist in removal of unreacted reactive dyes. However, heretofore, the liquid alkalis used during fixing have not been considered for use during soaping since they would not be expected to assist in excess dye removal. In addition, if the reaction mixture is too "hot" or alkaline, such as is seen with pure sodium hydroxide, the sensitive-type reactive dyes will hydrolyze with the water in the rinse bath and form a nonreactive pigment that has no effect on the fabric color.
Another liquid alkali which was recently introduced is sold under the tradename "REMOL FB". REMOL FB is available from Hoechst Celanese of Somerville, N.J. According to a chemical analysis, REMOL FB contains a mixture including about 30 wt. % potassium hydroxide, 10 wt. % anhydrous metasilicate, and the balance water. However, test dyeings have indicated that, like pure sodium hydroxide, REMOL FB is too "hot" for many classes of dyestuffs and, therefore, it would be expected to attack the dye during soaping off also.
Recently, there has been developed a new type of liquid alkali for dyeing with fiber reactive dyes which is based on potassium hydroxide and sodium silicate. This type of liquid alkali has been found to be suitable for a wide range of dyestuffs including those sensitive to "hot" mixtures. See copending application Ser. No. 07/954,589, filed Sep. 30, 1992, entitled "LIQUID ALKALI FOR REACTIVE DYEING OF TEXTILES". In addition, it has unexpectedly been found that liquid alkalis of this type may also be suitable for use during soaping off without causing the dye to hydrolyze with the water in the rinse bath.